1. Technical Field
The present invention concerns a method of drawing an optical fiber from the end of a glass preform which is heated to above the glass softening temperature at least in that area, while a gas flows around the preform during the drawing process and the subsequent feeding of the preform into a furnace space.
2. Description of the Prior Art
In one of the generic methods found in EP-A1-0 653 383, gas flows around the preform and the direction of the flow corresponds to the fiber drawing direction, while the gas flowing around the preform in the drawing region is stabilized by means of an additional flushing gas. This eliminates gas turbulence inside the furnace space, or limits its effect on the quality of the optical fiber drawn from the preform to an insignificant degree.
To improve the fiber quality and prevent diameter fluctuations in particular, a helium containing gas has already been guided in a downstream direction through the drawing furnace (DE-OS 29 06 071), at least during a part of the drawing process. This takes place in that a helium-free gas is first fed through the furnace in the drawing direction until the length of the blank is only about 10 cm, and then a helium-containing gas is used. Although the gas turbulence can be reduced in the furnace space because of the helium's higher heat conductivity and lower heat capacity as compared to other protective gases, the known measure however implies a considerable control engineering effort and uncertainty about the fiber quality during the gas change. This applies above all because of ever increasing requirements for higher production speeds and stricter specifications for the fiber diameter. Up to now in the field of optical fiber, requirements were made with respect to fiber diameters of 125 .mu.m. It was requested that this diameter should be within a tolerance range of .+-.2 .mu.m. Meanwhile, based on increasing requirements for higher production speeds and stricter specifications for the fiber diameter tolerance ranges of .+-.1.0 .mu.m are discussed based on online measurements at a high frequency.
To this must be added that more economy with improved fiber quality is demanded from the drawing process, requirements which cannot be achieved with the process cited last. Increased leakage can therefore always be expected on the inlet side of the drawing furnace, which results in an increased loss of helium if, as required to prevent diameter variations, a gas with a high helium content must flow from top to bottom through the furnace toward the drawing region.